Fiber-lasers are commonly pumped by light from a diode-laser. One preferred fiber-laser type that is suitable for diode-laser pumping is referred to by practitioners of the art as a double-clad fiber-laser or a cladding pumped fiber-laser. The double clad fiber-laser has a doped core that provides optical gain when energized by the pump light from the diode-laser. Surrounding the doped core is an inner cladding and surrounding the inner cladding is an outer cladding. The diode-laser light (pump light) is directed into the inner cladding of the fiber-laser and propagates through the inner cladding while being progressively absorbed in the doped core, thereby energizing (pumping) the core.
If a fiber-laser is required to provide a high power output, for example, greater than about 3.0 Watts (W), a single diode-laser emitter may not be capable of providing sufficient pump light power. It this case, it will be necessary to provide pump-light from a plurality of emitters. It is usually found convenient to provide an integrated linear array of such emitters or diode-lasers in what is termed a “diode-laser bar” by practitioners of the art. The emitters in the bar are preferably multimode emitters.
A multimode emitter usually has a higher power output than single mode emitter of the same length and heterostructure. The output power and the number of emitted lateral (spatial) modes of such an emitter usually increases as the width of the emitter increases. By way of example, a multimode emitter having an emitter width of 100 micrometers (μm) can emit as much as twenty or more times the power of a corresponding single mode emitter having a width of between 1 μm and 5 μm. In a common pumping arrangement, multimode radiation from a laser emitter is coupled into a multimode optical fiber. Light from the multimode optical fiber is, in turn, coupled to the fiber-laser.
Optimum absorption of pump light in a doped fiber core usually occurs in a relatively narrow band of wavelengths. By way of example, in a ytterbium (Yb) doped core, there is a strong absorption peak at a wavelength of about 977 nanometers (nm). The absorption peak has a full width at 90% maximum absorption (FWNM) of only about 1.0 nm. A diode-laser having a peak gain at 977 nm has a gain bandwidth of between about 4 and 6 nm. Accordingly, it is desirable that pump light have a wavelength equal to the peak absorption wavelength and have a bandwidth about equal to the peak absorption bandwidth.
In a diode-laser bar, lasing wavelengths of individual diode-lasers or emitters may be spread over a range of a few nanometers. Further, the individual emitters in the bar will exhibit a strong, temperature-induced wavelength shift. By way of example, for emitters nominally lasing at a wavelength of 977 nm, the wavelength variation with temperature change is about 0.3 nm per degree Kelvin (0.3 nm/° K). This relatively high temperature sensitivity, combined with the range of emitting wavelengths, makes a multimode diode-laser bar unsuitable for pumping a fiber-laser lasers wherein pump light must be absorbed in a narrow band of wavelengths. To provide an efficient absorption of pump light in a doped fiber core having a narrow absorption peak, wavelength locking or wavelength stabilization of diode-laser bars and narrowing of bandwidth is required.
Wavelength stabilization and relative insensitivity of the emitting wavelength to temperature change has been achieved, in a single-mode laser diode, by locking the lasing wavelength to the reflecting wavelength of a wavelength selective reflector arranged to form an external cavity or resonator for the diode-laser. The wavelength selective reflector is provided by a fiber Bragg grating (FBG) formed on a length of a single-mode fiber. Single mode radiation from the diode-laser is launched into the core of the single mode fiber and is partially reflected and partially transmitted by the FBG. The FBG typically has a reflection coefficient between about 0.5% and 8% at a wavelength near the peak gain wavelength of the diode-laser and has a reflection bandwidth of about 1 nm or less. The reflected radiation wavelength is defined by the optical period (hereinafter simply “period”) of the FBG. The emitting wavelength of the laser diode is locked to the peak reflection wavelength (resonance wavelength) of the FBG, and the emission bandwidth less than 1 nm. The resonance wavelength of an FBG is less sensitive to temperature change than the emitting wavelength of a (unstabilized) diode-laser. By way of comparison, the temperature sensitivity of the resonance wavelength for a FBG is about 0.01 nm/° K, while temperature sensitivity of lasing wavelength is about 0.3 nm/° K, as discussed above.
The FBG wavelength locking scheme is effective because the FBG is written in a single-mode fiber. In a single mode fiber, radiation is incident on the FBG at only one angle of incidence such that the wavelength of radiation reflected is determined only by the period of the FBG. Radiation from a multimode diode-laser must be coupled into a multimode fiber for efficient coupling. However, in a multimode fiber different modes propagate at different angles to the fiber axis. Were a FBG with fixed period written into such a multimode fiber, different lasing modes coupled into the fiber would be incident on the FBG at different angles, and, accordingly, would be reflected at different wavelengths. A result of this is that the output of the multimode diode-laser could not be locked to a single lasing wavelength. There is a need for a wavelength locking and stabilization scheme that is effective for a plurality of multimode diode-lasers the output of which is coupled into a plurality of multimode fibers.